Re: [ATM] ATM Edmund secondary mirrors - worth it?

["vladimir sacek" <vlad@copper.net>]

Surface error numbers themselves -either p-v or rms -
don't mean much when it comes to diagonal mirrors. 
What matters is the surface shape. A smoothly curved surface
will result in anywhere from near zero (for a specific toroidal shape)
 to about as much p-v astigmatism on the wavefront as is 
the nominal surface error. 

However, local surface error on the diagonal will be greater
 in the wavefront by a factor of ~2.8. This gives 
a good reason to go for smaller nominal errors, in general,
but still doesn't guarantee nothing. A "1/8 wave" diagonal
may perform better than "1/12 wave". The only reliable
indicator would be if the diagonal surface error is converted 
into an actual rms wavefront error. Not hard 
to do, but it would requre interferometricaly based procedure
for each single piece - probably the reason why it is not
practiced.

Vlad

----- Original Message ----- 
From: "Gil McFarlane" <gilboastro@yahoo.com>
To: "ATM" <atm@atmlist.net>
Sent: Sunday, April 17, 2005 2:24 AM
Subject: [ATM] ATM Edmund secondary mirrors - worth it?


> In looking for a 1.8"  secondary for a flex mirror
> scope, I found that Edmund has some on sale: $46 - but
> nothing is a bargain if it doesn't work well.  Any
> thoughts versus antares or protostar for about $75?  
> 
> Is it possible to see the difference between a 1.8 and
> a 1/12 wave optic?
> 
> Are these measurements valid?
> 
> Thanks,
> 
> - Gil
> 
> 
> 
> 
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